1. Field
The present invention relates generally to computer security and, more specifically, to content protection systems.
2. Description
Various mechanisms exist for protecting digital content when the content is distributed on a storage medium such as a compact disk read only memory (CD-ROM) or a digital versatile disk (DVD), or distributed via broadcast technologies. Typically, such mechanisms use some form of cryptography to protect the content.
In one system used for protecting content on DVDs, a single symmetric key is assigned to each manufacturer of DVD players. The DVD player uses the shared symmetric key embedded within the device to decrypt the content on the DVD and play it for a user. However, if one of the manufacturer's devices is attacked and the shared symmetric key is compromised, the key may be communicated to others to pirate the content. To attempt to stop this activity, the manufacturer may wish to revoke authorization of the key, thereby preventing further usage of the shared key. However, revocation of the single shared key would render all of the manufacturer's devices inoperable. Thus, this scheme does not practically support revocation of compromised keys.
Another system uses individual symmetric keys for each device. When a key is compromised, the compromised key may be added to a revocation list, or a broadcast encryption key block may be generated that revokes the compromised key. The revocation list or key block may be communicated to playback devices so that playback of the content by a device with compromised keys may be prevented. However, when many keys are compromised over time, the revocation list of compromised keys or a key block to be sent to playback devices may become very large and unmanageable. If the number of playback devices becomes very large (in the thousands or millions), then distribution of updated revocation lists or key blocks becomes problematic.
Content providers, broadcasters, and distributors must devise new methods for mass distribution of digital content to consumer devices in ways that deter piracy, yet do not include revocation mechanisms having undesirable side effects for the consumer.